Recently, illuminating fixtures using a semiconductor light emitting element such as a light emitting diode (an LED), an organic electroluminescence (EL) element, and the like, as a light source have been proliferated.
The type of illuminating fixture is provided with, for example, a lighting apparatus (an LED lighting apparatus) having a configuration as shown in FIG. 20 (see Document 1 [JP 2005-294063 A]).
This lighting apparatus includes a (first) switching element 92, an inductor 93, and a diode 95. The (first) switching element 92 and the inductor 93 are connected in series with a DC power supply 91. The diode 95 constitutes a closed circuit in combination with the inductor 93 and a light source load (light emitting diode) 94 while the switching element 92 is turned off.
This lighting apparatus is a self-excited type, and controls on-off operation of the switching element 92, thereby supplying electromagnetic energy stored in the inductor 93 while the switching element 92 is turned on, to the light source load 94 through the diode 95 while the switching element 92 is turned off.
In addition, this lighting apparatus includes a resistor 96 for measuring a current flowing through the switching element 92. The lighting apparatus varies on-duration of the switching element 92 in accordance with the current measured by use of the resistor 96. Thus, a current flowing through the light source load 94 can be kept constant by means of a self-excited type lighting circuit. However, the lighting apparatus in Document 1 is a self-excited type and does not have a dimming function. It is therefore impossible to dim the light source load.
Meanwhile, Document 2 (JP 2003-522393 A) discloses that supply power to a light source load (an LED lighting module) is turned on and off at a burst frequency of 100 Hz or 120 Hz synchronized with a frequency (50 or 60 Hz) of an AC power supply (a main power supply voltage).
The lighting apparatus (a power supply assembly) can control a length of a pulse in which the supply power to the light source load is in an On state, thereby performing a dimming control. However, a specific circuit configuration for dimming is not disclosed in Document 2.
However, as described in Document 2, in the lighting apparatus configured to perform dimming by controlling a pulse length (an On time), when a dimming ratio is small (dark), the On time in one period of the burst frequency is short, which may cause flicker. For this reason, in the lighting apparatus, a range of selectable dimming ratios is difficult to be set widely.